Supercooling characteristics of some deciduous fruit trees as related to water movement within the bud

Abstract

SummaryTo characterize and identify the freezing resistance mechanism of some deciduous fruit trees, woody stem and dormant buds of apple (Malus domestica ‘Tsugaru’), grape (Vitis labruscana ‘Muscat Bailey A’), peach (Prunus persica ‘Shimizu Hakuto’), Japanese pear (Pyrus pyrifolia ‘Nijisseiki’), and persimmon (Diospyros kaki ‘Hiratanenashi’) were used in the following experiments. On thermal analysis (TA), the woody stem tissues or the buds with woody stem tissue of all fruit trees used showed two exotherms, the high temperature exotherm (HTE) and the low temperature exotherm (LTE). The flower buds without woody stem tissue of peach and pear also had two exotherms, the HTE and LTE, and the LTE temperatures coincided with the LT50S of flower buds although the detection frequency of LTE in pear was considerably lower (<10%). Furthermore, in peach and pear, the LTE of buds without woody stem tissue and the LT50 of flower buds on woody stem were cooling-rate dependent, and those temperatures fell with a decrease in cooling rate. Water migration from the flower primordia of both buds to the surrounding tissues was also observed during freezing. In apple, although the LTE of bud without woody stem tissue was not detected in our experiment, the LT50 of the bud was cooling-rate dependent, and water migration from the primordia to the other tissues in bud was observed during freezing. These facts indicate that the mechanisms of freezing tolerance in apple flower buds are the same as in peach and pear flower buds. Contrarily, in persimmon and grape, flower buds without woody stem tissue showed only one exotherm whose temperature was closely associated with the LT50 of the bud. Also, both the exotherm temperature of bud without woody stem tissue and the LT50 of buds on woody stem were independent of cooling rate. Persimmon buds showed no changes in the water content of either the primordium or the surrounding tissue during freezing, indicating no water migration from the primordia to the surrounding tissues. In addition, provascular strands were found between the bud axis and flower primordia in apple, peach, and pear buds, but not in persimmon and grape buds. From these results, the buds of apple, peach and pear may acquire freezing resistance by extra-organ freezing, but grape and persimmon buds are likely to supercool by themselves.